The invention relates to an instrumentation amplifier intended for monolithic integrated circuit (IC) form. The amplifier is intended to operate in conjunction with an oxygen sensor to produce an output voltage that is related to the oxygen partial pressure in an atmosphere being sensed. The combination is of interest, for example, in automotive applications where the fuel-air mixture is to be precisely controlled so that engine performance can be controlled. This permits control of exhaust emissions. In order to control emissions, it is necessary that a relatively precise control be exercised over the fuel/air ratio supplied to the engine. This requires a sensor of suitable sensitivity and a precision instrumentation amplifier to buffer its output.
Generally speaking, an instrumentation amplifier is defined as an amplifier having differential inputs and a single ended output. Both input terminals operate at high impedance and the common mode rejection is high. The amplifier is linear and has well-defined gain. Typical operational amplifiers (op amps) cannot be used because their gain must be controlled with negative feedback which results in one input terminal having low impedance. It is common practice to employ a plurality of op amps coupled together and including negative feedback so that two high impedance differential input terminals are available, yet the gain is controlled with respect to a single output.
The automotive environment introduces a number of severe requirements for the sensor-amplifier combination. First a single power supply is used, usually the automotive battery, which has one terminal connected directly to the vehicle frame. While the sensor is also connected to the vehicle frame, its location is normally remote from the amplifier so that the two frame locations can be operating at substantially different potentials. For example, the sensor ground can be operating at a d-c potential of .+-.1.5 volts with respect to the amplifier ground. This represents a common mode range and almost any instrumentation amplifier can handle such a range with a very high degree of rejection. However, such amplifiers ordinarily require a bipolar power supply which is actually two power supplies with their common connection representing the nominal input potential level. Where a single power supply is used and one power supply terminal is grounded, the conventional IC operational amplifier (op amp) will readily reject common mode potentials that lie in a range extending from slightly above ground to slightly below the power supply potential. The exact range of common mode rejection is determined by the circuit configuration. In conventional IC construction, when the input is driven to a level more than one diode drop below ground the circuit becomes inoperative. For example, when a PNP input transistor is driven more than one diode drop below ground, the IC construction results in a forward biased diode coupled across the input terminals to ground. Excessive input current will flow and minority carriers are injected into the IC substrate where they can adversely affect other portions of the circuitry. When the input circuit employs NPN transistors, the high current does not flow as described above, but the stage becomes inoperative at some potential slightly above ground. Clearly such conditions are unacceptable in a system that can produce inputs greater than 1.5 volts below-ground.
The associated oxygen sensor also introduces problems that establish amplifier requirements that are uncommon. The circuit must cope with a sensor that produces a fractional volt output which varies with oxygen response and has an internal resistance that varies over several orders of magnitude as a function of operating temperature. The amplifier should produce a nominal output when the sensor is cold and then produce an oxygen related output as the sensor warms up during use.